1. Field of the Invention
The present invention relates to an optical frequency shifter using an electro-acoustic effect which has no insertion loss polarization dependency and no polarization mode dispersion, and to an optical soliton-like RZ (Return-to-Zero) pulse transmission system using such an optical frequency shifter which is suitable for an application to the optical submarine cables.
2. Description of the Background Art
The conventional optical frequency shifter using an electro-acoustic effect has a large insertion loss polarization dependency of about 0.5 dB for example, and a large polarization mode dispersion of about 10 ps delay for example, so that the conventional optical frequency shifter has been associated with a problem that the waveform deformation is caused for the high speed signals in random polarization states.
On the other hand, as a transmission system capable of realizing a very large capacity optical communication, there is an optical soliton transmission system using optical soliton pulses. In this optical soliton transmission system, an optical amplifier system using optical soliton pulses has various advantages over the usual optical amplifier system using non-return-to-zero pulses such as that a large capacity can be realized, that a multiplexing can be realized easily, and that there is a little degradation due to the non-linearity because the non-linearity of the optical fibers is utilized. For these reasons, the optical soliton transmission system has been studied very actively.
In the optical soliton communication, the optical amplifier noise affects the timing jitter of the optical pulses at a receiving terminal, and degrades the transmission characteristic. Namely, in the optical soliton to which the noise is superposed, there is a random jitter in the optical intensity and a shape of the optical soliton is slightly deformed from the ideal optical soliton shape, so that there arises a jitter in an amount of carrier frequency shift due to the non-linear optical effect. As this effect is repeated by repeaters, a random jitter in arrival times of the optical pulses is caused while the optical pulses are propagated through an optical fiber with a finite dispersion value. This phenomenon is called the Gordon Haus effect, which has been the major limitation of the transmission characteristic for the optical soliton communication.
In a case of transmitting a plurality of optical solitons having data, a shape of each soliton is not changed but adjacent solitons interfere with each other if an interval between adjacent solitons is narrow so that it is possible to observe a phenomenon in which adjacent solitons attract each other or repel each other. Such a phenomenon also causes a timing jitter at a receiving terminal, so that it is not desirable from a viewpoint of the application to the communication. In order to suppress the interference of the solitons, there is a need to provide a sufficiently wide interval between adjacent solitons.
In order to overcome the above described timing jitter, there have been active researches on the soliton control techniques for suppressing the timing jitter artificially, and the soliton transmission experiments have made rapid progresses in recent years. One available soliton control technique is a technique for controlling the random frequency shift in the frequency region by using the optical filter, and another available soliton control technique is a technique for directly controlling the timing jitter itself in the time region.
The control in the frequency region suppresses the timing jitter by bringing the random frequency shift which causes the timing jitter closer to the central frequency of the filter by using the optical narrow bandwidth band-pass filter provided behind the optical amplifier. This filter is called a frequency guiding filter as it guides the solitons which tend to move away from the center in the frequency region. The bandwidth of the filter is as narrow as 5 to 10 times the spectrum width of the soliton. Also, the noises will be accumulated in the frequency guiding with the fixed central frequency, so that there is a proposition of a method for slightly sliding the central frequency of the optical filter along distances, and such a filter is called a sliding frequency guiding filter.
This method is based on the principle that, the soliton components are non-linear waves that propagate while generating the frequency chirp by themselves so that the soliton components follow a slight change of the central frequency of the filter, but the noise components are linear waves which do not follow the filter frequency shift, so that the noise components are gradually set outside the filter bandwidth and the noise accumulation can be suppressed effectively. An amount for sliding the central frequency is approximately -6 GHz for 1000 km.
However, it is very difficult to apply these narrow bandwidth filters to the practical system for the following reason. Namely, the frequency of lights is approximately 200 THz but there is a need to shift an absolute value of the central frequency of the filter by about 200 MHz (which amounts to an accuracy of 0.0001% with respect to the central frequency) at each repeater in a case of the sliding frequency guiding filter, for example. However, in view of the current technological level and the environmental change such as a temperature change associated with the practical system, it is expected that such a minute control of the narrow bandwidth optical filter is nearly impossible in the practical system.
Also, when the application of the optical soliton communication to the optical submarine cables is taken into account, the conventional technique which requires a use of a very narrow bandwidth optical filter inside the repeater is not preferable because the repeater is required to have a high reliability from a practical viewpoint of the long term system reliability.
On the other hand, it is known that the effect effectively equivalent to the sliding frequency filter can be obtained by providing a frequency shifter and a fixed optical filter inside the repeater. In this case, the noises are set outside the bandwidth of the optical filter by the frequency shifter, but the optical solitons are trapped by the central frequency of the fixed filter.
However, the conventional frequency shifter has a large insertion loss polarization dependency and a large polarization mode dispersion so that there has been a problem that the conventional frequency shifter cannot be used in the practical system.